failureStrings property
If an item can't be purchased from the vendor, there may be many "custom"/game state specific reasons why not. This is a list of localized strings with messages for those custom failures. The live BNet data will return a failureIndexes property for items that can't be purchased: using those values to index into this array, you can show the user the appropriate failure message for the item that can't be bought.
Implementation
@JsonKey(name:'failureStrings')
List<String>? failureStrings;